Defects in masks have always been a source of yield reduction in integrated circuit manufacture. As the minimum sizes approach 1 .mu.m and below, and the circuits are designed with higher device densities, defects that were once tolerable can no longer be accepted. Common sources of defects are incorrect design of the mask patterns and flaws introduced into the patterns during the pattern generation process. Because each mask is printed on large numbers of wafers, fatal defects in a mask are highly undesirable. It would be useful if such defects could be detected and repaired prior to printing.
Device features are primarily fabricated using photolithography. The art of photolithography embodies techniques for creating two-dimensional patterns on a work surface by the controlled application of energy (such as electromagnetic, ion beam or other radiation) to a reactive material deposited on a wafer. In a photolithographic process the energy application is controlled through the use of a patterned photomask. The pattern is transferred to a photoresist coating on the wafer surface, forming a resist pattern. The wafer is then etched according to the resist pattern and, following the etch, subjected to further processing steps. The resulting features are the basis of the final circuit. As can be seen, the accuracy of the mask pattern and the resist pattern play important roles in the quality of the circuit. As area and feature size decreases, the impact of pattern defects and optical effects increases proportionately. Defects in either the mask or resist pattern during processing may have a direct affect on the accuracy and electronic characteristics of the semiconductor device.
Mask fabrication defects have a variety of causes. Such causes include, but are not limited to, defects in the original substrate, introduction of particulate matter during fabrication, scratches, or improper processing. In an attempt to minimize the number of defects introduced during wafer processing, photomasks are inspected after they are created and before they are used to pattern the wafers. Conventional inspection procedures examine several characteristics of the mask, including line width measurement, measurement of the pattern registration, whether all features present in the design database have been transferred to the mask, and whether any mask fabrication defects have been produced while manufacturing the mask. Current systems employ different inspection tools and methods for each of the above inspections. Originally, inspections were carried out by a human operator. As masks have become more complex this task has been relegated to automatic detection systems which perform the task more rapidly, with better sensitivity and repeatability and with fewer errors.
Some conventional inspection systems reduce material costs by comparing an image of the mask to the original data. U.S. Pat. No. 5,481,624, issued to Kamon et al., entitled "Mask Inspecting Method and Mask Detector," describes an inspection method similar to that disclosed in U.S. Pat. No. 4,641,353, issued to Kobayashi, entitled "Inspection Method and Apparatus for a Mask Pattern Used in Semiconductor Device Fabrication." Kamon's method is directed to phase shifting masks, which include extra features in the mask to account for the unique optical effects of the phase shifting material used to manufacture the mask. Both Kobayashi and Kamon expose the actual pattern embedded in the mask using the same optical conditions as those used in a wafer exposure and compare that to the original pattern data. These methods are an attempt to detect defects in the pattern before it is printed on wafers.
In a conventional die-to-database system such as that described in Kobayashi, the data defining the original pattern is compiled and prepared. A photomask is then fabricated using the original pattern data. The conventional inspection system acquires a two-dimensional image from the photomask and conditions the resulting image. Conditioning the two-dimensional image cleans it up and enhances the image for future processing. The original pattern data is reformatted into a two-dimensional binary image acceptable to the inspection system. The reformatted data image is then converted to gray scale and filtered to resemble an acquired two-dimensional image. The two images are then aligned, and any discrepancies between the two images are flagged as potential defects. A conventional die-to-die inspection system works in a similar fashion. The primary difference is, instead of formatting one set of data from the original pattern data, mask data from two pattern images acquired from the mask are compared to each other.
Conventional inspection systems detect defects in any one of three images: that defined by the original data from which the mask is constructed; the pattern after it is printed on the mask; or the pattern after it has been printed on the wafer. In conventional systems, any inspection at a given stage of the process will potentially pick up anomalies introduced at that or earlier stages. However, using conventional inspection tools, many defects are not noticeable until the feature is produced in three dimensions by forming the pattern in the resist, due at least in part to the fact that defect printability in the resist is a function of the exposure tool and of the resist characteristics. Defects which appear at this stage, however, are more costly to repair. When defects are discovered prior to resist processing, only a single mask need be repaired or replaced. Defects not discovered until after the resist is formed are likely not found until they have been replicated over large numbers of wafers. All of the affected wafers must then either be repaired or discarded. What is needed is a way to anticipate these less obvious defects before resist processing begins.
Existing inspection methods are limited because they are unable to anticipate the defects which appear when the resist is formed on the patterned wafer. Such defects result from defects in the pattern as well as from characteristic behavior of the expose tool or the resist material during processing. Existing methods do not take into account the characteristics of the expose tool or the resist material which will be formed according to the mask pattern. As a result, a mask may be inaccurately flagged as defective where, even though the mask pattern and original pattern are not identical, the "defect" would not impact the final resist pattern. Alternatively, there may be subtle mask defects that are not captured using conventional inspection techniques, but which cause resist defects due to the characteristics of the exposure tool and the resist material. The mask is, as a result, inaccurately flagged defect-free, when in fact one or more defects will appear when the resist is formed according to the pattern. What is needed is a way to accurately identify "true" defects at a point where they can be corrected or avoided at lower cost.
Systems which do not identify defects until after the pattern has been printed on the wafer increase process costs because each defect is likely repeated over a number of wafers before it is discovered. What is needed is a reliable way to determine, prior to forming the resist, whether resist formed according to a particular pattern will contain any defects. Such a system would reduce production costs.